Current biochemical and structural data suggest that DNA polymerases from a variety of organisms use similar catalytic mechanisms for the synthesis of DNA. A more precise understanding of the reaction mechanisms of these proteins and the identification of structural features that dictate their substrate specificity, processivity, and other enzymatic properties await high resolution structures of DNA polymerases in catalytically-competent complexes with DNA and nucleotide substrates. The existing DNA polymerase structures are limited to enzymes that are adapted for distributive short-gap synthesis and, with one exception (Pol P), these structures are missing one or both substrates. We are investigating the structure of T7 DNA polymerase, its interaction with substrates, and its association with other proteins of the DNA replication fork complex.